1. Field of the invention
The present invention concerns the manufacture of a transparent polymer material article with a refractive index gradient whereby the refractive index varies continuously in at least one direction.
It is more particularly, although not necessarily exclusively, directed to the situation in which the article can be used to manufacture an ophthalmic lens.
It is known that varying the refractive index of an ophthalmic lens makes it possible to reduce the maximum thickness and therefore the weight so that the same correction can be achieved with greater comfort for the user, for example.
Various methods have already been proposed for obtaining a refractive index gradient in a polymer material.
2. Description of the prior art
As disclosed in American patent US-A-3.718.383, for example, an additive composition, in practice a dilutant, can be absorbed into the polymer material, a hardened polymer material in this instance, in practise by diffusion.
Alternatively, as disclosed in the Japanese patent document No 60-175009 a gel can be treated with a monomer adapted to produce a polymer whose index of refraction is different than that of the base material, after which the combination is finally polymerized to set the geometry of the refractive index gradient which is then obtained.
However, this geometry depends directly on the concentration profile of the monomer within the polymer base material.
It therefore depends on the polymer/monomer interaction and can therefore be affected by changes of temperature that may occur during the diffusion process.
Under these conditions, it is difficult to control accurately the spatial distribution of the refractive index gradient obtained.
In either case, a refractive index gradient can be achieved only over a limited distance, this being the distance over which the monomer diffuses into the polymer base material.
To obtain accurate control of the spatial distribution of the refractive index gradient it has been proposed to diffuse a mixture of monomers into a gel, to vary the composition of the mixture during the diffusion process until a relative concentration profile is obtained which matches the required refractive index gradient profile and then to complete the polymerization of the whole to fix the resulting refractive index gradient geometry.
Although satisfactory in other respects, this method is unable to increase significantly the distance over which a refractive index gradient can be obtained, this distance being still limited by the diffusion process.
A general object of the present invention is a method enabling this difficulty to be overcome and the spatial distribution of the refractive index gradient obtained to be monitored and controlled.